In 2368 Ambassador Spock secretly traveled to Romulus to promote Romulan-Vulcan reunification and uncovered a plot to invade Vulcan.
In 2368 Starfleet learned that Ambassador Spock had vanished and might have defected to the Romulans. Picard and Data, disguised as Romulans, traveled to Romulus and found that Spock had gone there on his own to work with an underground movement that wanted the Romulan and Vulcan peoples to reunite.
Spock's contact, Senator Pardek, turned out to be working with Sela, who had used the reunification movement as cover. Romulan forces had stolen Vulcan ships and planned to send them to Vulcan with an invasion force, pretending to be a peace delegation.
Picard, Data and Spock broadcast a warning that stopped the invasion, and Sela's ships were destroyed by her own side when they were exposed. Spock decided to remain on Romulus to keep working toward reunification with the underground. Before leaving, Picard shared a mind-meld that allowed Spock to feel a connection to his father, Sarek, who had died shortly before.